Non-aqueous electrolyte batteries, particularly lithium primary batteries, have been used as power sources for electronic equipment such as portable devices. Such electronic equipment is designed to be used in an environment having a temperature ranging from about −20° C. to 60° C. on the basis of the human living environment.
However, in recent years, the equipment using batteries has been applied in a wider range, and in association with this, the range of the operating temperature of such equipment tends to be widened. For example, primary batteries that can keep functioning for a certain period of time even when the operating environment temperature is supposed to be 125° C. at maximum and can operate at a temperature as low as about −40° C. have been required as batteries to be used in vehicle-mounted apparatus.
Typical lithium primary batteries include CR batteries in which manganese dioxide is used as a positive electrode active material and BR batteries in which fluorinated graphite is used as a positive electrode active material.
In general, CR batteries are excellent in load characteristics at low temperature but are insufficient in high-temperature characteristics. Specifically, at a high temperature of 60° C. or higher, the non-aqueous electrolyte is decomposed by catalysis of manganese dioxide in the presence of a very small amount of water in the battery, causing gas to be generated. As a result, the battery swells and the bonding tightness in the interior of the battery is reduced, which may greatly increase the internal resistance in the battery.
On the other hand, BR batteries are excellent in high-temperature characteristics because the reactivity between materials such as the reaction between the fluorinated graphite and the non-aqueous electrolyte is low even at a high temperature of 100° C. or higher, and the increase in internal resistance in the battery is small. For this reason, at a high temperature of 100° C. or higher, BR batteries are more reliable than CR batteries.
The fluorinated graphite that is currently used as a positive electrode active material for a lithium primary battery is fluorinated graphite having a high content of fluorine, since such fluorinated graphite has a high capacity and is excellent in the flatness of the discharge voltage. BR batteries using such fluorinated graphite are very excellent in high-temperature characteristics, but are insufficient in discharge characteristics at low temperature, and thus, for example, the discharge rate characteristics may be deteriorated.
Moreover, in the case of using fluorinated graphite, the voltage drops sharply at the beginning of discharge and then shows a slight increase, which is followed by an almost constant voltage, and then the voltage is stabilized. Such a sharp drop in voltage causes no problem in the case of a discharge at a very weak current, for example, in the case of being used as a memory backup power source, since the drop in battery voltage is small. However, in the case of being used in equipment for transmitting electric waves such as a tire pressure sensor of vehicle-mounted apparatus, since the operating current is large, the drop in voltage, particularly the drop in voltage at the beginning of discharge may be increased.
Further, in the future, if the equipment is miniaturized, the size of batteries would need to be reduced following the miniaturization of the equipment. The reduction in size of batteries involves a reduction of the reaction area in electrodes and an increase of the current density of discharge, and as a result, the discharge voltage is further reduced.
In order to improve the discharge characteristics at low temperature, for example, one proposal suggests forming a hydrophilic functional group on the surface of fluorinated graphite by a method selected from ozone treatment, plasma treatment, corona treatment, and ultraviolet irradiation treatment (see Patent Literature 1). According to this proposal, the hydroxyl group or carboxyl group formed on the surface of the fluorinated graphite ameliorates the wettability of the positive electrode with non-aqueous electrolyte, and thus ameliorates the low-temperature discharge characteristics at −20° C.
Patent Literature 2 discloses a method of irradiating fluorinated graphite with γ-rays in order to prevent an appearance of a minimum voltage point at the beginning of discharge and to reduce the drop in voltage at the beginning of discharge. The irradiation partially dissociates the C—F bond on the fluorinated graphite surface, forming a carbon layer on the surface.
Patent Literature 3 discloses a method of irradiating fluorinated graphite with ultraviolet rays while the fluorinated graphite is impregnated in or wetted with an organic solvent. According to this method, the fluorinated graphite surface is partially defluorinated, and a carbon layer is formed on the surface.
Non-Patent Literature 1 discloses a method of heating fluorinated graphite at 300 to 450° C. in a hydrogen gas atmosphere to dissociate the C—F bond on the fluorinated graphite surface, and thus forming a carbon layer on the surface.
In Patent Literatures 2 and 3 and Non-Patent Literature 1, a carbon layer is formed on the fluorinated graphite surface to impart conductivity to the fluorinated graphite. This can prevent the appearance of a minimum voltage point at the beginning of discharge.
In order to ameliorate the discharge rate characteristics, Patent Literature 4 suggests using fluorinated graphite having a particle size of submicron order. Patent Literature 4 discloses that, by using a powder of fluorinated graphite having an average particle size of 1 μm or less, the surface area of the fluorinated graphite is increased, and the occurrence of the reaction through which the fluorinated graphite absorbs lithium is allowed to increase, and as a result, the discharge characteristics are ameliorated.
Patent Literature 1: Japanese Laid-Open Patent Publication No. 2006-59732
Patent Literature 2: Japanese Laid-Open Patent Publication No. Sho 58-5966
Patent Literature 3: Japanese Laid-Open Patent Publication No. Sho 58-26457
Patent Literature 4: Japanese Laid-Open Patent Publication No. 2005-247679
Non-Patent Literature 1: N. Kumagai, et. al., J. Applied Electrochem., vol. 20 (1995), page 869-873